The light emitting diode (called LED) was invented in 1960. The LED was commercialized until 1998 from the red light LED, the green light LED and the blue light LED. The LED has functions of less power consumption and fast switch. The LED is applied in indicating area, for example, the traffic signal and the flat-panel display. The LED is applied in a new area after the LED is being a backlight in high-level cellphone.
Presently, the LED is applied in many areas, for example, the remote control, the motor vehicle, the lighting system, and the outdoor large display etc. Besides, the LED is being a potential product of the display due to its high color saturation, fast switch, no mercury and long life so that the LED can be applied in the display light source and the back light.
Please prefer to FIG. 4, it is a schematic block diagram showing a circuit of a conventional lighting system. The lighting system 400 comprises a driving device 406 and a light source 408. The lighting system 400 receives an alternating current voltage supplied by the alternating current voltage source 402 through the rectifier 404. The driving device 406 comprises a switch unit 412, a feedback unit 416, a control unit 418, a timing synchronous unit 420 and an illumination control device 422.
The timing synchronous unit 420 transfers the alternating current voltage into a timing synchronous signal. The control unit 418 is electrically coupled to the timing synchronous unit 420 for outputting an adjustment signal to the switch unit 412 according to a timing of the timing synchronous signal. The switch unit 412 is electrically coupled between the rectifier 404 and the light source 408. The switch unit 412 decides to supply the alternating current voltage to the light source 408 or not according to a state (i.e. high logic level or low logic level) of the adjustment signal when the switch unit 412 receives the adjustment signal. Therefore, the light source 408 will be enabled. The feedback unit 416 is electrically coupled between the light source 408 and the control unit 418 for detecting a loading state (for example, a current value of the light source 408) of the light source 408 and outputting a feedback signal to the control unit 418 according to detecting result.
A predetermined illumination value is obtained from the illumination control device 422 after the feedback signal is received by the control unit 418. Users according to the user's requirement can adjust the predetermined illumination value. Then, the control unit 418 compares the feedback signal with the predetermined illumination value for reference of width of the adjustment signal.